Endothelial Regulation of Calmodulin Expression in Vascular Smooth Muscle Cells

Interactions between vascular endothelial cells and the underlying smooth muscle cells (VSMCs) are of paramount importance in maintaining vascular functions. Calmodulin (CaM) is involved in a wide variety of cellular functions and is a limiting factor in both cell types, with free cytoplasmic CaM constituting only a small fraction of the total cellular CaM. Currently nothing is known about potential interactions between ECs and VSMCs as it involves CaM. We have begun to investigate the possibility that vascular endothelial cells impact VSMC functions via CaM‐dependent activities. Using a co‐culture model of primary culture vascular endothelial cells and smooth muscle cells isolated from the same vessels, we have found that VSMCs in co‐culture with proliferating ECs express on average 90% more CaM than monocultured VSMCs. Media fractionation and eNOS inhibition experiments indicate that the CaM‐elevating effect was exerted by a soluble factor that is not nitric oxide. The CaM‐elevating effect exerted by ECs is strongly dependent on endothelial density, such that at a starting 50% confluency of VSMCs, a starting 20% endothelial confluency triggers the greatest CaM increase in VSMCs, whereas a starting 70% endothelial confluency yields no visible effect on VSMC CaM expression after 48 hrs. Pharmacological inhibition of cyclooxygenase‐1, vascular endothelial cell growth factor (VEGF), and endothelin‐1 receptors (ET A and ET B ) does not affect the observed increase in CaM. The data suggest that proliferating endothelial cells produce a soluble factor that can regulate CaM‐dependent signaling in VSMCs via alterations in total cellular CaM expression.